The present invention relates to the transmission of radio frequency (RF) communication signals. More particularly the invention relates to a method and apparatus for transmitting RF communication signals in multiple frequency bands.
Multiple communications standards have driven the need for wireless communication devices such as cellular and personal communication service (PCS) radiotelephones to be compatible with multiple standards. Having multimode capability allows one device to operate with more than one system or standard and, depending on the standards available, the user can potentially use the device on more than one continent or more than one country. A multi mode communication device is designed to transmit and receive RF communication signals of either an analog or a digital nature or a combination thereof depending on the communication systems in operation in the users geographical location. The standard for transmitting analog RF communication signals typically the advanced mobile phone system (AMPS) and the transmission standard for digital RF signals may be chosen from a plurality of multiple access techniques including time division multiple access (TDMA), code division multiple access (CDMA), and global system for mobile communications (GSM). A multimode device incorporates electronics necessary to operate within a plurality of these standards, for example a dual mode device has the capability to operate in two systems, following two different standards. This allows the user to move from one system to another and the device will operate as long as the multimode communication device incorporates one of the standards for the system in operation. The transmission portion of a multimode communication device is similar to those devices which are single mode, or designed to transmit within one frequency band, but are adapted to transmit at multiple frequency bands in accordance with any of the standard protocols mentioned above, analog or digital.
Typical single band RF transmitter circuitry comprises a power amplifier (PA), a and pass filter, impedance matching circuitry, and an antenna. The PA amplifies the communication signal in accordance with the desired communication protocol. Impedance matching circuitry minimizes the RF loss as the signal passes form the PA to the antenna. The filter allows the RF communication signal of the desired frequency band to pass through to the antenna and to transmission in to the air interface. The band pass filter and impedance matching circuitry are generally designed specifically to operate in a given frequency band. In order to transmit in multiple adjacent or separated bands, additional components are required to allow frequency band adjustments.
One method for achieving multiband transmission capability is to employ two or more individual PA devices into the wireless communication device. This requires one PA for each mode or frequency band that the device is designed to operate in. This further requires providing a corresponding number of electrical paths, one for each individual PA and related circuitry for filtering and impedance matching. In this scenario, a first PA and related circuitry would be optimized to operate in a first frequency band in accordance with any of the analog or digital standards. A second PA and related circuitry would be optimized to operate in a second frequency band in accordance with the desired standard. This approach requires minimal design effort however, having multiple PA""s adds significant cost to the product as PA""s are one of the more expensive components due to their complexity. In addition, the cost and complexity are exacerbated with PA""s used for digital RF transmission.
A second method for achieving multiband transmission capability is to utilize a pin diode circuit. With the pin diode circuit a single PA is followed by a pin diode switching network embedded into the input and output impedance matching network. The pin diode switching network controls the post amplifier impedance matching of the communication signal in response to electrical command signals sent by the microprocessor. Changing the impedance characteristics of the pin diode switching network changes the frequency response of the circuit in accordance with the operation mode of the device. For example, when the communication device is in a first operation mode, the pin diode switching network is switched to the first RF band corresponding thereto. This method however requires an extensive number of parts increasing both cost and complexity of circuit, and rendering optimization and tuning very difficult. Also because the pin diodes are nonlinear, these circuits tend to distort the desired communication signal and generate adjacent channel power (ACP) noise thereby interfering with users on neighboring channels. This makes their utilization in the linear modulation schemes (CDMA and TDMA) prohibitive.
A third method utilizes a diplexing circuit following a single PA which provides a first path and a second path, one path for each mode of the dual mode communication device. The first path is designed for a first RF band in accordance with a first mode and a second path, designed for a second RF band in accordance with a second mode. In this configuration the PA output is connected to a signal transmission line that diverges into the first path and the second path. Each transmission line is connected to a band pass filter and related matching circuitry corresponding to the desired band.
In order for the diplexing method to work, the circuit must include long signal transmission lines to carry the amplified signal between the PA and the components of and the respective filtering. A first consequence is that, space on the printed circuit board (PCB) must be available to accommodate the necessarily long transmission lines as both the line length and width are critical to the circuit performance. Second, the necessary length of the transmission line makes the circuit extremely lossy requiring more power and making the overall circuit less efficient.
A variation of the diplexing method removes the issue of long, lossy transmission lines, but another disadvantage prevails. This circuit requires high precision discrete components to achieve the high Q value necessary for proper circuit performance in place of the lengthy transmission line. This does allow for reduced space necessary on the PCB for the transmission line but the number of high precision parts increases significantly adding to the cost of the device.
A final disadvantage to the diplexing method is the critical out of band impedance of the post PA filters. In order for the diplexing to work properly, the out of band impedance of the first band filter must be accurately known when the circuit is operating at the second frequency band and conversely the out of band impedance of the second frequency band filter must be accurately known when the circuit is operating at the first frequency band. This is necessary to ensure that the transmission lines are the proper length and that the PA sees an open circuit at the second RF band while looking into the first RF band path. Specifying that the out of band impedance be precise, adds cost and complexity to the filter and overall network design.
The use of additional circuitry poses a problem as this requires more space within the device and cost reduction is almost always desired. The current methods are inefficient creating unwanted power drain and reducing overall circuit performance. Therefore, there is a need to improve the means and method of transmitting RF signals in multiple frequency bands using fewer components while maintaining or improving the current transmitter performance levels.